Coaxial line r.-f. choke



Oct. 11, 1955 M. B. HALL 2,720,631

COAXIAL LINE R-F CHOKE Filed Dec. 21, 1945 INVENTOR MAURICE B. HALL ATTORNEY United States Patent COAXIAL LINE R.-F. CHOKE Maurice B. Hall, Cambridge, Mass., assignor, by mesne assignments, to the United States of America as represented by the Secretary of the Navy Application December 21, 1945, Serial No. 636,474

12 Claims. (Cl. 33397) This invention relates to radio frequency impedance elements and more particularly to radio frequency chokes for guided wave transmission lines.

The conventional finger type R.-F. (radio frequency) choke as used in a coaxial transmission line has a number of disadvantages, such as, difficulty encountered in adjustment, a tendency to shift its position when heated, or shocked and material from fingers accumulating in cavity due to shifting position for adjustment.

In accordance with the present invention and as is explained more fully hereinafer, the above-mentioned difiiculties are eliminated.

A principal object of this invention is to provide in a guided wave transmission line means to permit the passage of no power beyond a particular point almost all power being reflected therefrom.

Another object of this invention is to provide in a coaxial transmission line by means of a combination of a dielectric member of predetermined dimension, a conductive hollow cylindrical member of predetermined diameter and an air passage an impedance which precluded the flow of power beyond this impedance.

A further object of this invention is to provide in a coaxial transmission line by means of a combination of a dielectric member of predetermined dimensions, a conductive cup choke of predetermined dimensions and an air passage an impedance which precludes the flow of power beyond this impedance.

A still further object of this invention is to provide in a guided wave transmission line by means of a combination of the above mentioned elements an attenuator which will allow the passage of a limited amount of power.

These and other objects will be apparent from the following specification when taken with the accompanying drawings, in which:

Fig. 1 is a longitudinal section of a coaxial transmission line with the novel radio frequency impedance element shown in position.

Fig. 2 is a cross section of the coaxial transmission line of Fig. 1 taken in the plane IIII of Fig. 1.

Fig. 3 is a longitudinal section of a coaxial transmission line of a second embodiment of the invention.

In Figs. 1 and 2 an embodiment of the invention is illustrated. The dielectric element 12, which comprises a substantially cylindrical member having a central, axial opening, is fitted within cylindrical outer conductor of the coaxial transmission line. A cylindrical metallic element 13 is fitted to the inner surface of the aforementioned central opening in the dielectric element 12 and thus separates the dielectric element 12 from the airfilled portion 14 surrounding inner conductor 11 of the coaxial transmission line.

The metallic element 13 between the dielectric element 12 and the air-filled portion 14 of the coaxial line makes it possible for the incident electric wave being transmitted through the line to propagate independently in dielectric 12 and air-filled portion 14. The diameter of 2,720,631 Patented Oct. 11, 1955 metallic element 13 is such as to allow substantially equal amplitudes of the incident electric wave to propagate independently in the two portions. The dielectric element 12 will produce a considerably shorter wavelength than will the air-filled portion 14 of the guide. The length of the dielectric element 12 along the longitudinal axis of the coaxial line is chosen so that there will be a one-half wave length diflerence in the path lengths of the incident wave in the two portions. A part of the incident electric wave indicated by the arrow in Fig. 1 will be reflected upon striking the front edge 21 of the member 12, part will pass through member 12 and part will be reflected from the back edge 22 of member 12. The device may also be so dimensioned and the dielectric 12 so chosen that the wave reflected from the front and back edges 21 and 22 of dielectric 12 will reinforce. The choice of dimensions and dielectric constant is made roughly as follows:

It is recalled that thewave reflected from the back edge 22 of the dielectric element 12 (a boundary where a medium of high dielectric constant is followed by a medium of low dielectric constant) will at that point experience no phase shift. Suppose, for example, that the dielectric constant of dielectric element 12 to be nine, and that the length of dielectric element 12 is three quarters of a wave length in the dielectric. The length will then be one quarter of a wave length in the air-filled portion 14 of the guide (wave length is inversely proportional to the square root of the dielectric constant). The incident waves travelling through both air space 14 and dielectric 12 will then cancel at the plane of rear edge 22 as desired.

The reflected wave from the rear edge 22 of the dielectric 12 will experience a one and one-half wave length shift during the round trip through the dielectric element 12. The reflected wave from the front edge 21 of dielectric 12 experiences a 180 phase shift, because the boundary represents a medium of low dielectric constant followed by a medium of higher dielectric constant. Hence the reflected waves from the front and rear edges 21 and 22 of dielectric element 12 reinforce, as desired,

causing the power to be reflected back down the guide in a direction opposite to the propagation of the incident wave designated by the arrow in Fig. 1.

In Fig. 3 a second embodiment of the invention for a coaxial transmission line corresponding to that in Fig. l. and having an outer conductor 19 and an inner conductor 11. This embodiment is designed to reduce the frequency sensitivity or (broad band) the impedance element. The region between inner conductor 11 and outer conductor 10 is divided into three sections. Path 40 is the air-filled separation, which would ordinarily be small, between the metallic cup choke 16 and the outer conductor 10. Some or" the incident energy will pass directiy down path 40. Some of the energy, going down path 20, will be reflected from the far end of metallic cup choke 16 and will arrive at the front edge 50 of the impedance element, out-of-phase with that which went directly into path 40. Thus that part of reflected wave from path passing down path 4% will eifectiveiy reduce the amplitude of that part of the incident wave going directly down path 40.

Some of the energy from the incident wave will go directly down path which is filled with cylindrical dielectric element 15 positioned upon the inner conductor 11. Some of the reflected power from cup 16 will also pass down path 30, producing an elfect in path 30 similar to that in path except that in path 30 the wave length will be different in order to produce the desired phase shift for interference at the rear edge of the impedance element.

By proper dimensioning, the total energy which passes through path 40 the air-filled portion and path 30 the dielectric element 15 can bemade equal in magnitude, and by proper choice of the dielectric constant of element 15, along with proper choice of dimensions, these two waves can be made to be out-of-phase when they arrive at the rear edge; 60 of the impedance element. Thus the device is an effective radio frequency impedance element. a A still further modification is to fill the metallic cup choke 16 with a dielectric material (not shown) of a different dielectric constantfrom dielectric 12 in path 30., With appropriate choice of dimensions and dielectric constants any desired degree of radio frequency at-.

tenua tion may be obtained.

Thus, it is to be clearly understood that the description and illustrations of the invention made above have been given only by way of example and not as a limitation on the scopeof the invention as set forth in the objects and the accompanying claims.

What is claimed is:

1. A radio frequency choke for a coaxial transmission line having a substantially cylindrical outer'conductor and an inner conductor, comprising a substantially cylindrical dielectric member adapted to fit within said outer conductor, a cylindrical and axial opening in said dielectic member and a hollow cylindrical conductive member secured to said dielectric member and within said opening, the diameter of said hollow cylindrical conductive member being such as to allow substantially equal amounts of energy flow through said opening and through said dielectric member, said dielectric and hollow members being of selected length such that the radio frequency energy passing therethrough, respectively, is 180 out of phase thereby to effect substantially-complete reflection thereof;

2. Apparatus as in claim 1 wherein the axial length of said dielectric member is related to that of said hollow member to provide a half-wave length difference of effective path such as to produce a cancellation of energy propagation beyond a more'remote boundary edge thereof and reinforcement of energy propagation reflected Y froman incident boundary edge thereof, thereby to enhance the reflectivity and minimize the transmissive characteristics of said choke.

3. A radio frequency choke for a coaxial transmission line having a substantially cylindrical outer conductor and an inner conductor, comprising a substantially cylindrical dielectric member adapted to fit over said inner conductor, a conductive cup choke secured to and surrounding said dielectric member, said cup choke comprising a pair of conductive hollow cylinders conductively joined at one edge thereof, said cup choke being sepatromagnetic energy, said line having an enclosing outer conductive member, an impedance element for reflecting substantially all of said energy of predetermined frequency incident thereon comprising, a material of high dielectric constant symmetrically disposed within said outer member, said material intercepting a portion of said electromagnetic energy and shifting the phase thereof to a predetermined excess over at said frequency, the unintercepted remainder of said energy being unaltered in phase by said material, said material further having a length in said line .so related to said dielectric constant that said excess of phase shift equals the phase change within said unintercepted remainder, thereby to effect complete stoppage of said energy at the impedance element. 7

5. In a cylindrical coaxial transmission line, an impedance element for reflecting electromagnetic energy incident thereon comprising, concentrically disposed annuli of materials of different dielectric constant, said annuli air-filled annulus, each concentric with said line and longitudinally coextensive therealong, each having coplanar entrant and emergent faces perpendicular to said line and an electrical shorting annulus member coextensive with and between said annuli whereby said energy is separately conducted within said annuli, the length and dielectric constant of the annuli being mutually related to provide a half-wave length greater path in the dielectric annulus; V

7. A totally'reflecting stop for a radio frequency coaxial line comprising concentric insulating rings longitudinally coextensive within said line having selected differing transmission velocities for radio energy such that Within said rings a half-wave length difference in'contained standing waves persists, the cross-sectional areas of the rings. being selected for substantially equal energy transmission, whereby energy passing through said stop is substantially totally reflected within the stop.

8. A coaxial line stop according to claim 7 wherein the standing wave within one said ring is of one-quarter wave length and in the other said ring is three-quarters wave length. r

9. A coaxial line stop according to claim- 7 wherein one said ring is air-filled and the other said ring is of solid dielectric material. 7

10. A coaxial line stop according to claim 7 including spaced cylindrical conductive separators between said rings, said separators having an annular opening therebetween in the direction of arriving radio energy and closed-circuit interconnection in the opposite direction, thereby forming an energy-reflecting stub.

11. A coaxial line stop according to claim 7 including a cylindrical metallic separator between said rings.

12. A coaxialline stop according to claim 11, said cylindrical separator having an'air-filled annulus therein of substantially one-quarter wave length open'in the direction toward incident radio frequency energy.

References Cited inthe file of this patent UNITED STATES PATENTS 2,129,669 Bowen Sept. 13, 1938 2,180,950 Bowen Nov. 21, 1939 2,299,619 Fritz Oct. 20, 1942 2,407,911 Tonks Sept. 11, 1946 Collard Feb. 27, 1951 

